The HELLAS2XMM survey - XIII. Multi-component analysis of the spectral energy distribution of obscured AGN

摘要

Aims. We combine near-to-mid-IR Spitzer data withshorter wavelength observations (optical to X-rays) to get insight intothe properties of a sample of luminous, obscured active galactic nuclei(AGN). We aim at modelling their broad-band spectral energydistributions (SEDs) in order to estimate the main parameters relatedto the dusty torus that is assumed to be responsible for thereprocessed IR emission. Our final goal is to estimate the intrinsicnuclear luminosities and the Eddington ratios for our luminous,obscured AGN. Methods. The sample comprises 16 obscured high-redshift (), X-ray luminous quasars (ergs-1)selected from the HELLAS2XMM survey in the 2-10keV band. Theoptical-IR SEDs are described by a multi-component model that includesa stellar component to account for the optical and near-IR emission, anAGN component that dominates in the mid-IR (mainly emission from adusty torus heated by nuclear radiation), and a starburst to reproducethe far-IR bump. A radiative transfer code to compute the spectrum andintensity of dust reprocessed emission was extensively tested againstour multiwavelength data. While the torus parameters and the BHaccretion luminosities are a direct output of the SED-fittingprocedure, the BH masses were estimated indirectly by means of thelocal -relation. Results. The majority (80%) of the sources show moderate optical depth (), and the derived column densities are consistent with the X-ray inferred values (cm-2)for most of the objects, confirming that the sources are moderatelyobscured Compton-thin AGN. Accretion luminosities in the range ergs-1are inferred from the multiwavelength fitting procedure. We comparemodel luminosities with those obtained by integrating the observed SED,finding that the latter are lower by a factor of 2 in the median. The discrepancy can be as high as an order of magnitude for models with high optical depth ().The ratio between the luminosities obtained by the fitting procedureand from the observed SED suggest that, at least for type2 AGN,observed bolometric luminosities are likely to underestimate intrinsicones and the effect is more severe for highly obscured sources.Bolometric corrections from the hard X-ray band are computed and have amedian value of .The obscured AGN in our sample are characterized by relatively low Eddington ratios (median ). On average, they are consistent with the Eddington ratio increasing at increasing bolometric correction. Key words: quasars: general - galaxies: nuclei - galaxies: active